Correct way to service 2024 l8t allison trans?

[Aimatdeer]

I have a 2024 silverado 2500hd with the l8t gasser and 10 speed. Not quite 10k miles on it. I don't tow or haul a lot. A little bass boat and a 5600# camper 6-7 times a year. Manual calls for 45k trans service. I am going to do them every 40k. My question is the best/correct way to service. My dealer says pan drop and filter change. A dealer nearest me says they don't drop pan unless there is a problem. They hook lines up and I don't know if it's a flush or a transfer of all the fluid. I have always heard mixed things about a flush. So what is the best thing for the trans?

 

[AndrewF]

I would drop the pan as it’s worth changing the filter. Then replace with a pan that includes a drain plug to make the process easier next time and allow for fluid only changes. Dorman has a factory capacity pan with a plug or there are a few aftermarket higher capacity expensive ones. 

[Aimatdeer]

I probably will have a dealer do it. I just want to choose who will do it right. Any gm techs here that know correct procedures? While it's under warranty i don't want to change anything.

[Ry Guy]

The correct procedure is:

1) drop the Y-pipe under the transmission pan, 

2) unscrew the transmission pan bolts to let the fluid drain out, 

3) pull the pan and gasket, 

4) replace the filter, 

5) clean the magnet, 

6) reinstall the pan (the gasket is reusable assuming it is in good condition) and reattach the y-pipe, 

7) slightly overfill the transmission through the fill port on the side, 

8 ) get the transmission temp up to 167-176F by doing converter stalls (10 seconds running the engine up to 1500 rpms with the brake applied, followed by 10 seconds off idle - repeat until the temperature is in the range), 

9) unscrew the fluid level check bolt with the transmission temp in this range and let the fluid drain out until there is only a steady drip coming out,

10) reinstall the fluid level check bolt.

 

If fluid does not come out at step 9, turn the engine off, let the area around the transmission cool down, add additional fluid through the fill port, and repeat steps 8-10.

 

 

 

Edited March 15, 2025 by Ry Guy

[64BAwagon]

Dorman makes a replacement pan for around $150 (Rock auto) that has a drain plug or Superior makes a good drain plug kit for a little over $20

The hardest part will be pulling the Y pipe. If I do mine I will probably loosen the trans mounts bolts and jack it at the tailshaft for clearance. 

[carkhz316]

Regarding the pan to exhaust clearance concern: A carefully placed prybar between the crossover and the frame with a gentle push/ pull will afford you the clearance to remove the pan. It may require a second set of hands though. This is much quicker and easier than removing the exhaust or anything else.

[Pryme]

17 hours ago, 64BAwagon said:

Dorman makes a replacement pan for around $150 (Rock auto) that has a drain plug or Superior makes a good drain plug kit for a little over $20

The hardest part will be pulling the Y pipe. If I do mine I will probably loosen the trans mounts bolts and jack it at the tailshaft for clearance. 

I can’t remember reading if the six speed has the exhaust in the way or not? Do you recall on your 6spd?

[Chuck FB]

2 hours ago, carkhz316 said:

Regarding the pan to exhaust clearance concern: A carefully placed prybar between the crossover and the frame with a gentle push/ pull will afford you the clearance to remove the pan. It may require a second set of hands though. This is much quicker and easier than removing the exhaust or anything else.

I was wondering if you were referring to your 2021 model with the 6 speed or the original posters 2024 model with the 10 speed as I am only guessing there is going to be difference in the relationship of the exhaust and clearance to slip the trans pan out. I look at my truck with the 10 speed and its hard to visualize the pan coming out without some more clearance but only someone having attempted it hands on would one know for sure. I can certainly see why any Duramax aftermarket deeper style pan would never fit on 2024+ L8T truck setup and they always say "fits Duramax only" etc.

[64BAwagon]

19 hours ago, Pryme said:

I can’t remember reading if the six speed has the exhaust in the way or not? Do you recall on your 6spd?

Unfortunately Yes.  If I recall correctly its not nearly as tight as the Fake Allison but its still in the way. 

 

Edited March 17, 2025 by 64BAwagon

[mrjulian416]

Plenty of clearance on my '21 6 speed to get the factory pan off and install a cast aluminum pan. 

[No F-bdy Bs]

About the time mine needs service, I expect the exh studs to be rusted just enough to break the studs off. 

[64BAwagon]

Flame wrench

 

[Chuck FB]

11 hours ago, No F-bdy Bs said:

About the time mine needs service, I expect the exh studs to be rusted just enough to break the studs off. 

I wonder if it would do any good to take each nut off and slather the stud in high temp anti seize ( doing them one at a time assuming the pipes would stay locked in place so as not to disturb them ). In fact does anyone know if those connections use a donut seal insert or is it just a steel on steel precise fit ?

[Chuck FB]

On 3/16/2025 at 3:18 PM, 64BAwagon said:

Unfortunately Yes.  If I recall correctly its not nearly as tight as the Fake Allison but its still in the way. 

 

I'm sure if I was to get custom lettering made up " Fake Ass Allison" ( magnetic stickers since the truck is steel after all ) and had parked it out front of the Chev dealer customer entrance, they might not be impressed

[No F-bdy Bs]

12 hours ago, Chuck FB said:

I wonder if it would do any good to take each nut off and slather the stud in high temp anti seize ( doing them one at a time assuming the pipes would stay locked in place so as not to disturb them ). In fact does anyone know if those connections use a donut seal insert or is it just a steel on steel precise fit ?

I've considered doing that. Definitely not a bad idea, especially if your roads see salt. 30min and some anti-sieze could save a lot of grief in the future.